In the United States, buildings are responsible for almost half of all annual greenhouse-gas emissions and consume about three-fourths of the electricity generated by power plants. That, coupled with the fact that the majority of the buildings in which we will live and work over the next 30 years have not yet been built means we have significant opportunity to reduce the carbon footprint of buildings and stem climate change.

The sustainability of a new building is based on many factors, including water savings, energy efficiency, and materials selection. These factors are influenced heavily by a building's architectural, site, and building-systems design and supporting civil infrastructure. Mechanical engineers can support sustainable design by providing input about green approaches early in the design process, designing more efficient and better-sized mechanical systems, and producing metrics and supporting documentation for evaluation and green certification when needed.

This article will describe how building-information modeling (BIM) and BIM-based design tools enable mechanical engineers to simulate, analyze, and document their designs more efficiently and accurately and ultimately deliver greener building systems and healthier, more resource-efficient buildings.

Drivers for green design are numerous and include owner demand and a growing attitude toward environmental stewardship within the building industry, not to mention a host of government regulations and green-building incentives. The federal government, as well as many states and local communities throughout the country, have initiated programs and enacted legislation regarding green-building design. For example, the Energy Policy Act of 2005 provides financial incentives for sustainable building. Title 24, Part 6, Energy Efficiency Standards for Residential and Nonresidential Buildings, of the California Code of Regulations sets minimum energy-efficiency standards for all new homes, additions to and alterations of existing homes, and most commercial buildings. New York City Local Law 86, also known as the Green City Buildings Act, requires that new municipal buildings and additions to and renovations of existing municipal buildings meet green-building standards.

Standards and rating systems for green-building systems abound. In the United States, ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, is a widely used standard providing minimum requirements for energy-efficient building systems in new and renovated buildings. The U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED) Green Building Rating System is the most prevalent scorecard used by those pursing green-building certification. Of the 69 total LEED credits available, approximately 25 percent are influenced by building-systems design.

Internationally, the trend toward sustainable-building design also is strong. Governments around the globe are implementing new building regulations that mandate sustainable design. Many countries already require performance assessments to comply with building regulations. Additionally, countries around the world have or are adopting voluntary rating systems similar to the LEED rating system, such as Green Globes (Canada), Building Research Establishment Environmental Assessment Method (England), Comprehensive Assessment System for Building Environmental Efficiency (Japan), and Green Star (Australia).

Although the building industry's (and owners') interest in sustainable design is undeniable, it has its challenges. Some issues are technical, while others relate to standard industry processes and practices. Cost always is a concern. However, the growing market demand for sustainable design is outweighing and overcoming these hurdles and driving fundamental process changes throughout the industry. Transformative concepts that facilitate sustainable design, such as integrated project delivery and BIM, quickly are becoming the standard.

BIM

BIM is an approach to building design involving the use of a digital building model created from coordinated, consistent design information enabling whole-building analysis, faster decision-making, and better documentation.

BIM software offers many benefits for general building design. The best BIM software uses a centralized, parametric model allowing "live" viewing and automatic coordination of all plans, quantity takeoffs, and other related documentation. These integrated deliverables have explicit relationships with each other and the model, resulting in better-coordinated construction documents that minimize errors and omissions.

The design model is used for a variety of building analyses, automatic clash detection, design visualizations, and precise quantity takeoffs. In addition, the resulting digital design model can be leveraged for a variety of related tasks, such as construction sequencing, digital fabrication, and facilities management.

Perhaps the greatest advantage of BIM in sustainable building design is building analysis. Sustainable building design hinges on the ability to gain insight into a building's performance through design analysis and optimization. But evaluating building performance based on the building representations produced by conventional computer-aided-design (CAD) or object-CAD solutions requires a great deal of human intervention and interpretation and makes the analyses unduly time-consuming and costly.

With BIM, much of the data needed to support performance analysis is captured naturally as design proceeds. With BIM, designers can analyze how a building will perform, even in the early stages of design. Armed with this information, they can evaluate design alternatives quickly and make better decisions for greener designs. By streamlining design and analysis, BIM facilitates the calculations needed to optimize building performance.

A BIM-based design model carries a wealth of information necessary for many other aspects of sustainable design. For example, the ability to create drawings and details directly from a model (and have the software automatically coordinate these drawings and details with the model) improves the efficiency and accuracy of green certification. Schedules of building-material quantities can be obtained directly from a model to determine percentages of material reuse, recycling, and salvage. Various design options for sustainability can be pursued in parallel and automatically tracked in a model. Advanced visualization techniques can be used for solar studies and to produce 3-D renderings and construction animations of a green project. A digital 3-D model supports better understanding and collaborative communication among the various stakeholders in a green partnership (the architect, owner, consultants, review bodies, etc.).

To begin building-systems design, a mechanical-engineering consultant leverages the architectural design model. By using the architect's model, the mechanical engineer ensures that the building-mechanical-systems design and model are coordinated, eliminating a redundant modeling effort to recreate the architect's building geometry.

The mechanical engineer defines all of the heating/cooling spaces and zones, adds information--such as the number of people per room, the heat load from equipment in the room (for example, the number of computers), etc.--and exports that model to an XML file. In addition to space and zone information, this file captures building geometry and other information, such as lighting density, sensible- and latent-load contributions, building-construction thermal properties, desired room-temperature set points, required ventilation airflow, cooling-coil temperature, heating-coil temperature, etc. As such, the file represents an accurate thermal model of the project.

The file then is imported into an analysis package, which determines the building's energy usage and heating- and cooling-load calculations. With their own design model leveraged directly for analysis, mechanical engineers avoid the time-consuming, error-prone task of manually entering data into an analysis solution.

Once the analysis is done, the resulting data can be viewed in a report and exported to a BIM-based mechanical, electrical, and plumbing (MEP) design model. For example, all of the heating- and cooling-load requirements for each space are exported to the MEP design model, enabling the mechanical engineer to view the information via the BIM software and use the software's calculations to size equipment, ductwork, piping, etc.

The mechanical engineer then can use "what-if" design scenarios, such as changing the R-values of various walls, to see how the changes would affect the total energy usage of the building.

Some BIM software includes built-in analysis tools that can be used to accurately predict a building's peak heating and cooling loads. This allows engineers to quantify needed airflow and properly size HVAC equipment, ensuring that energy is not wasted powering oversized equipment.

This design/analysis/optimization workflow is typical of BIM-based practices. Analysis packages with tighter integration are on the horizon. For example, some BIM software platforms feature programmatic links--which do not require export/import to a neutral file format--to analysis-software solutions. By further streamlining design/analysis workflows, these integrations facilitate conceptual-stage inline analysis and enable more complete information pathways. In addition, even enhancements to construction techniques, such as streamlining direct digital fabrication of ductwork to reduce material waste, are becoming more prevalent.

Sustainable Design in Practice

Design West Engineering is a full-service MEP consulting firm based in San Bernardino, Calif. Established in 2000, the firm specializes in mechanical-, electrical-, and telecommunication-engineering applications and energy-efficiency projects for a range of building sectors, including education, medical, civic, residential, and commercial.

To facilitate a new level of project collaboration with its architectural clients and structural engineers and transform its sustainable-design practices from ad-hoc to technology-based, Design West adopted BIM software in early 2007. The firm has completed construction documentation on 12 projects using BIM.

One of Design West's current projects is a new $110 million, 340,000-sq-ft educational facility for Coachella Valley Unified School District in Indio, Calif. Scheduled for occupancy in the fall of 2011, the facility will house approximately 3,700 middle- and high-school students. The facility will consist of nine structures and include 104 classrooms, an administrative building, two gymnasiums with basketball courts, an outdoor swimming pool with a changing facility, and an outdoor stadium with two concession facilities. The campus is being designed to earn points under the Collaborative for High Performance Schools (CHPS) High Performance School Recognition and Rating Program, a sustainable-design rating system for K-12 schools in California. Therefore, the entire campus design will be analyzed rigorously to increase its energy performance.

"A large portion of the target CHPS points are related to energy efficiency," Joel Londenberg, a project manager for Design West, said. "So our mechanical design must be analyzed in the context of how the building envelope is constructed, including windows, walls, roofs, and so on."

For this project, the entire design team, including the MEP firm, architect, and structural-engineering firm, collaborated on a single BIM platform. Design West is able to leverage the architectural and structural project models for its building-systems design, as well as for cross-discipline clash detection and coordination.

"It's also possible to use the architect's model in conjunction with energy-analysis software to optimize the building-energy performance early in the design phase of the project," Londenberg said.

For example, Design West's engineers need to properly account for daylighting and its effects on the heating, cooling, and lighting requirements of individual spaces in the facility. The desert climate of Coachella Valley, Calif., makes it particularly important to balance the desire to bring light into a classroom with the need to keep heat out. An accurate digital building model integrated with energy-analysis tools greatly simplifies this daylighting analysis and allows Design West engineers to size the cooling system properly and perform the compliance calculations needed to meet building codes.

"This sophisticated level analysis in the design phase cannot be achieved easily without BIM," Londenberg said. "By using BIM, we're able to consider the building in greater detail earlier in the design, which allows for a more thorough design process and provides a new level of design coordination and collaboration that was never possible before."

Conclusion

Growing awareness of the impact of buildings and infrastructure on the environment has increased the need for building-industry professionals to embrace sustainable practices. Sustainable design is a major trend driving process change within our industry, requiring a workflow that provides more information earlier in the design process. BIM is poised to facilitate this change because it enables an integrated design workflow, linking design and analysis.

As the use of BIM in the building industry grows, building designs and outcomes will become more accurate, buildable, predictable, and sustainable, enabling the cost-effective design and delivery of healthy, resource-efficient buildings and mitigating the carbon footprint of our built environment.

Robert E. Middlebrooks, AIA, is an industry manager with Autodesk Inc., a provider of building-information modeling and technology for the architecture, engineering, and construction industries. An architect with more than 26 years of experience and formerly a principal of a 360-person architectural/engineering firm, he has led design projects, including collaborative design-build and developer-led integrated projects, in more than 14 countries in Europe, Africa, the Middle East, and the Caribbean.